Almond and walnut trees often require some pruning to select scaffolds or to facilitate orchard management or harvest. Unfortunately, resulting wounds open the cambium layer and expose the tree to fungal and/or bacterial infections. Large pruning wounds on younger trees can have a big impact on their future health if the cuts become infected.
This dormant season’s early dry weather in most growing regions has given growers a larger window for pruning and removing dead wood and infected tissues in order to minimize inoculum sources and reduce the chances for infections next spring and summer.
However, wet weather has made a much-needed return this month, and growers should be aware of the risk of pruning wound infections, especially in young orchards, says Franz Niederholzer, University of California Cooperative Extension farm advisor in Colusa, Sutter, and Yuba counties. Proper timing of pruning activity is critical to preventing infections, he says.
If possible, young trees should be pruned in dry weather, when rain is not in the forecast. Rain within two weeks of pruning can lead to an increase in risk of microbial infections, as wounds provide an opening into the tree for rain- or wind-carried spores. Infections can be slow to grow and may not develop into full-blown canker diseases — or they can be aggressive and rapidly kill wood.
Ceratocystis, Eutypa, Botryosphaeria sp., and Phytopthora fungi can all cause canker in almonds after infecting pruning wounds. Some of these cankers are slow growing, but they may become perennial cankers that eventually girdle and kill branches. Phytophthora cankers can expand from an infection site at pruning wounds to more than 6 inches within three weeks of infection. As spring progresses, amber colored gumballs extruding through the bark are frequently seen at the cankers margin.
University of California researchers Themis Michailides (walnuts) and Florent Trouillas (almonds) have industry-funded research under way studying the fungal pathogens responsible for pruning wound infections, and evaluating fungicides to prevent or suppress pruning wound infections. Their work is providing data to support new labels for pruning wound protection.
A recent post on thealmonddoctor.com, the nut crop website developed and managed by David Doll, UCCE farm advisor in Merced County, reported that two products, Topsin M WSP and Rally 40WSP, both trademarked names, now have 2EE FIFRA labels for use to protect pruning wounds from canker disease infection for almonds and certain stone fruits. FIFRA is the Federal Insecticide, Fungicide and Rodenticide Act — the federal statute that governs registration, distribution, and sale of pesticides in the U.S.
PROTECTING YOUNG TREES
These products should be helpful in protecting young trees where infection of relatively large pruning cuts can occur and compromise long-term tree health, he says. Pruning so cut surfaces can be easily treated will improve product efficacy. Most protectant fungicides can wash off with rain and/or degrade with time, so applications may need to be repeated.
Recommendations on the labels include application within 24 hours of pruning, ensuring complete coverage of all cut surfaces, and adding a registered spray dye to better assess coverage. The 2EE labels also recommend use of an organosilicone surfactant to improve pesticide penetration into the pruning wound surfaces.
There should also be an additional fungicide application after two weeks, especially if there is a rain event forecast or high humidity. Topsin M is a Fungicide Resistance Action Committee 1 fungicide and Rally is a FRAC 3; both have single site activity.
Doll also notes differences on the product labels that limit how they can be used — separately or combined. The Rally label does not allow painting the product on pruning wounds, while the Topsin label does allow painting.
Labeled rates are also different. The maximum use for Topsin is 3 pounds per acre per year, and Rally use is at a maximum of 1.5 pounds per season. If they are combined in a tank mix, painting is not allowed, and only two applications per season are allowed. [Always read the label when preparing to apply a pesticide, and follow the label when using the product.]
The 2EE labels (current as of September 2017) can be read online.